How to Save Your Dahlias From Spider Mites

Now is a great time to get spider mites under control, even if you aren’t seeing any signs of them.

If spider mites have been a problem in the past, chances are they’re lurking nearby, just waiting for the warm, dry weather to roll in. Early prevention gives you a big leg up—and here’s why it’s totally worth the effort:

1. Less Bloom, Less Risk: If your dahlias aren’t blooming yet, you’ve got fewer pollinators visiting. That means fewer worries about accidentally spraying your bee buddies while trying to kill mites.

2. An Ounce of Chill: You know the saying—"an ounce of prevention is worth a pound of trying-not-to-sob-while-spraying-every-other-day." Catch mites early and you can stick with gentler methods: water sprays, soaps, and maybe a round of predatory mites. No chemical panic needed.

3. Skip the Panic Sprays: Waiting until you see bronzed, stippled leaves means you’re already behind—and playing catch-up usually involves heavier-duty stuff and more frequent sprays. Prevention = less stress and fewer interventions.

4. No Egg Left Behind: Mites reproduce ridiculously fast—like, new generation in a week fast. By starting early, you break the cycle before it starts. No eggs = no explosions later.

5. Cleaner Plants, All Season: Early action helps keep your foliage cleaner and happier throughout the season. Healthier leaves = better photosynthesis = better blooms. (Science!)

6. Buy Yourself a Breather: Want to take a summer weekend off from the garden? Early prevention gives you a buffer so you’re not out there in July rage-spraying at dawn.

Spot the Signs

Spider mites damage plants by piercing leaves and sucking out their juices. Early signs include stippling (tiny pale dots), but as the damage ramps up, you’ll see bronzing and eventually leaf drop. That leaf loss can lead to sunburn on exposed stems and disrupt both vegetative growth and flower production. Other symptoms include curled or scorched leaf edges and a tough, leathery leaf texture. Once populations build, you’ll spot fine webbing across leaves, buds, and stems—kind of like nature’s warning tape.

Spider mites love hot, dry weather and congregate under leaves. Watch for:

• Yellow speckles or mottling on leaves

• Fine webbing under foliage

• Leaves turning brown or dropping early

• Use a white sheet or paper and tap leaves—movement or tiny red/green specks = mites!

Level 1 Defense: Hose 'Em Down (The Gentlest Approach)

Level 2 Defense: Soap 'Em Down (Let Them Know You're Serious)

When rinsing isn’t enough, step it up. Horticultural oil is effective when used at the very first signs because it interrupts the reproductive cycle. For these to work, you need to soak the back side or underside of the leaves where the spider mites reside.

Horticultural oil smothers spider mites and eggs  but has no long-term effect. Reapply in 7-8 days after the first application to get get any survivors, and the do a third application in 5-6 days after the second application.

• Avoid use if rain is expected within 24 hours
• Horticultural oil should not be used during the day if temperatures are in the 90's or on stressed plants.
• This protocol should be limited to twice each season to avoid plant stress.

Recommendations 

• Monterey Horticultural Oil 

• Bonide All Seasons Horticultural Oil & Dormant Oil Spray  

Level 3 Defense: Bring in the Troops (But Don't Wait Until It's Too Late)

Natural predator allies love to munch on mites:

• Predatory mites like Phytoseiulus persimilis

• Ladybugs and lacewings 

Release them early to help balance the ecosystem, but know that most sprays later will harm your beneficials. 

Level 4 Defense: Miticides and Other Approaches (Use Wisely)

If the infestation persists, miticides kill adults, not eggs. Rotate chemical classes to reduce resistance. Introducing a miticide before your garden is in full bloom can reduce the impact to pollinators. Also, knowing when to spray can reduce the impact, too.

Captain Jack's Dead Bug Brew (Spinosad) is a first step into the chemical world. Spinosad does not impact the eggs of a spider mite. Each course should include three treatments no more than 10-12 days apart. No more than two courses should be applied each season.

NOTE: Just a heads up - organic doesn't always mean pollinator-safe. Captain Jack's Dead Bug Brew is an Insecticide and Mite Killer produced by Bonide that is safe for Organic Gardening. It uses spinosad, which is derived from a naturally occurring soil bacterium. Spinosad is still highly toxic and is only safe for bees when used in accordance with package directions. To minimize bee exposure, instructions say to apply when bees are not active, like very early in the morning or very late in the evening. 

Venerate XC & CG is an OMRI listed bioinsecticide with multiple modes of action. It’s toxic when ingested, breaks down insect exoskeletons, and disrupts molting—all of which can kill target pests on their own. But combined, they deliver serious knockout power. These varied modes of action also make it harder for pests to develop resistance, and you can adjust the mix depending on the density of the spider mite population. This can be applied every 3-10 days. If you plan on using a chemical to get mites under control and then releasing beneficial insects, beneficial insects can be released after 48 hours of use. If you are a home grower with fewer than 50 plants to spray, this can be challenging to mix. Check out BioAdvance 3-in-1 for another option.

BotaniGard Maxx uses a living fungi and adds pyrethrins for a quick knockdown. This product delivers multiple modes of action to to stop populations before they explode. This product also works to control aphids, thrips, June bugs, stink bugs, flea beetles, earwigs, and more. If you are a home grower with fewer than 50 plants to spray, this can can be tricky to mix in small batches or without a backpack sprayer. Check out BioAdvance 3-in-1 for another option.

BioAdvance 3-in-1 can be a great solution for a home gardener. It is easier to mix than Venerate and Botanigard Maxx. This product comes in a ready-to-use spray bottle and a concentrate that is easy to mix even for home gardens. This product offers disease, insect, and mite control and kills on contact. This is an easy-to-use but aggressive chemical so use wisely.

Prevention, Care, Aftermath

One of the most effective organic tactics against many spider mites is simply removing their favorite habitat—old foliage. Strip aging leaves, bag them in plastic, and toss them in the trash—never leave them in the garden or compost pile. After topping a dahlia, once new laterals grow a few sets of leaves, remove any remaining foliage from the main stem. When you cut a bloom, take off the lower lateral leaves too.

If a plant shows heavy mite damage, pull it and toss the whole dang thing or chop that sucker back if there's still time for new growth. Remember, if you just throw it in the compost pile, you aren't removing the mites. 

By the time you see the typical yellowing from mites, the pests have likely moved upward and left only eggs behind. A heavily infested plant won’t bounce back—it’s more of a mite factory than a flower producer and will only spread the problem.

Staying on top of foliage removal can do more to control mites than most chemical sprays. 

Healthy plants = mite-resistant plants:

• Strip off and discard old, infested foliage—don’t compost it

• Keep dahlias well-watered and mulched.

• Start mite sprays ~60 days after planting and keep an eye out all season

• Avoid planting near dusty paths or asphalt that heats up too much.

You’ve Got This

Spider mites multiply fast, but so can your garden resilience. With smart monitoring, a strong rinse, targeted sprays, and ecological balance, your dahlias will be back to their show-stopping selves. Want help picking out predatory mites or choosing which miticide fits your vibe? Just say the word!

With dirt & sweat,

Rebecca

1. American Dahlia Society – Coping With Spider Mites 

2. University of Illinois Extension – rinse technique

3. Colorado State & CSU – oil & soap control info

4. The Spruce (2025) – diatomaceous earth, predators, neem thespruce.com

5. University of Connecticut - Horticultural Oils